1. Field of the Invention
This invention relates to electret materials, and more particularly, to an electret speaker and a method of manufacturing the same.
2. Background of the Invention
An electrostatic speaker operates on the principle of Coulomb's law that two conductors with equal and opposite charge may generate a push-pull force between them. The push-pull electrostatic force may cause vibration of a diaphragm, and thereby generating sound. An electrostatic speaker may typically include two porous electrodes and a diaphragm placed between the electrodes to form a series of capacitors. The electrodes and the diaphragm may be separated by air gaps to provide space for the diaphragm to vibrate. The diaphragm is usually thin and light, and thus making the electrostatic speaker superior to other types of speakers, such as dynamic, moving-coil or piezoelectric speakers, with respect to its transition response, expansion capability in high frequency, smoothness of sound, acoustic fidelity and low distortion.
With the simple structure, electrostatic speakers may be manufactured in various sizes to accommodate increasing demand for small and thin electronic devices. However, a conventional electrostatic speaker requires a DC-DC converter to provide high voltage to the speaker. Considering the size, cost and power consumption of DC-DC converters, electret materials have been developed to replace DC-DC converters. An exemplary electret speaker is illustrated in FIG. 1, which may include porous electrodes 6a and 6b, and a diaphragm 4. The electrodes 6a and 6b may have a number of openings 61a and 61b on each electrode having a porosity of at least 30 percent. The electrodes 6a and 6b may be made of metals or plastic materials coated with a conductive film. The openings 61a and 61b may be provided for allowing sound waves to pass through. The diaphragm 4 may include a conductive layer 2 sandwiched between electret layers 1a and 1b. The electret layers 1a and 1b may contain either positive charges or negative charges. The electrodes 6a and 6b, and diaphragm 4 may be held in place by holding members 5a and 5b. The holding members 5a and 5b may be made of insulating materials. The electrodes 6a and 6b are separated from the diaphragm by insulating elements 51a, 51b, 61a and 61b. In operation of an electret speaker, each signal source 7a and 7b outputs an equal and opposite alternating signal to the electrodes 6a and 6b via conductive lines 8a and 8b. The signals cause a time-varying electric field to develop between the electrodes 6a and 6b and the electret layers 1a and 1b, thus resulting in a push-pull force. The push-pull force may cause the diaphragm 4 to vibrate. The resultant sound waves may pass through holes 61a and 61b to generate sound.
However, for an electret speaker to enhance its acoustic fidelity and low distortion, it requires an electret material with excellent charge storage stability and also a delicate process to fabricate a thin electret-metal-electret structure. It is known that fluorine-containing polymers, such as poly(thtrafluoroethylene) (PTFE), and fluorinated ethylene propylene (FEP), may have superior capability of electric charge storage. However, these materials may not adhere well to metals and are not suitable for being fabricated into a thin-film structure. Some fluorine-containing solutions such as CYTOP from Asahi Company and Teflon AF 1600 from Dupont Company, are expansive and not suitable for fabrication of diaphragms due to their machining property. As for other types of polymer electrets, such as polystyrene (PS), polycarbonate (PC), polyvinyl chloride (PVC), polymethylmethacrylate (PMMA), it is known that they may possess charge storage capability and may be dissolved in solvents, such as toluene, xylene or p-xylene. High density polyethylene (HDPE), polypropylene (PP) may be dissolved in p-xylene at a temperature of about 120° C. Polyimide (PI) and polyetherimide (PEI) may be dissolved in N-Methylpyrrolidone (NMP) or Dimethyformamide (DMF). In 1997, it was discovered that cyclic olefin copolymer (COC) possesses better electret and water-repellant property. Also COC may be dissolved in toluene, xylene and p-xylene to form a polymer solution. These polymer solutions mentioned above may be applied to fabricate single-sided diaphragms due to its superior machining property. However, their charge storage capability is not good enough for electret speakers and they may have adhesion issues on forming an electret-metal-electret structure.